Method for producing composite metal wire

ABSTRACT

A method is provided for cladding aluminum on steel wire in controllable thickness to produce a composite metal wire at high rates of productivity. Steel wire is rapidly advanced through the center of a melt extrusion orifice during a concomitant extrusion of molten aluminum through the orifice. As the steel coated with hot, molten aluminum emerges from the orifice, contact is made with a gaseous atmosphere that is reactive with the aluminum. Immediate reaction occurs and a thin film is formed about the aluminum coating which serves to stabilize the coating against break-up caused by surface tension forces until solidification can occur through heat transfer.

nited States Patent Hobo [ Oct. 22, 1974 1 1 METHOD FOR PRODUCING COMPOSITE METAL WIRE Emerick J. Dobo, Cary, N.C.

[73] Assignee: Monsanto Company, St. Louis, Mo. 22 Filed: June 4, 1973 [2]] Appl. No: 366,920

[75] Inventor:

Primary ExaminerR. Spencer Annear 5 7] ABSTRACT A method is provided for cladding aluminum on steel wire in controllable thickness to produce a composite metal wire at high rates of productivity. Steel wire is rapidly advanced through the center of a melt extrusion orifice during a concomitant extrusion of molten aluminum through the orifice. As the steel coated with hot, molten aluminum emerges from the orifice, contact is made with a gaseous atmosphere that is reactive with the aluminum. Immediate reaction occurs and a thin film is formed about the aluminum coating which serves to stabilize the coating against break-up caused by surface tension forces until solidification can occur through heat transfer. 1

6 Claims, 4 Drawing Figures METHOD FOR PRODUCING COMPOSITE METAL WIRE BACKGROUND OF THE INVENTION This invention relates to composite metal wire and methods for producing it. More particularly, the invention is concerned with an efficient and commercially practical method for producing aluminumclad steel wire.

Because of severe fluctuations in both the supply and price of copper, a need has developed for electrical conducting wire which is comparable to that of copper wire in performance but constituted of cheaper material. Aluminum wire would appear to meet this need in view of the greater supply of aluminum as compared with that of copper and the fact that it is a better conducting material on a unit weight basis. However, such potential has not to date been fully realized largely because aluminum does not possess the tensile strength and fatigue resistance required for many electrical conductor applications.

It has been recognized that these property inadequacies could be compensated for by combining aluminum with another metal of high strength. This has led to attempts to clad aluminum on to a steelcore wire to produce a composite wire conductor. However, for the most part, methods previously proposedfor accomplishing this have notbeen commercially practical. In addition to an inadequate productivity, they have not been generally successful in controlling or attaining the desired thickness of aluminum on the steel base wire.

It is therefore, an object of this invention to provide an efficient method capable of high productivity for producing a composite metal wire of aluminum with an inner steel core.

It is a further object of this invention to provide a method for cladding aluminum on to solid steel wire in a controllable depth of thickness to produce composite metal wire having a wide range of applicability as an electrical conductor.

It is a still further object of this invention to provide the capability for producing aluminum-clad steel core wire composites wherein the aluminum cladding constitutes the major proportion of the total crosssectional area of the composite wire.

SUMMARY OF THE INVENTION US. Pat. No. 3,658,979 incorporated herein by way of reference as an adjunct to this description sets forth the basic precepts by which fibers and filaments may be formed by the melt extrusion of those materials which exhibit little if any significant viscosity when in the molten state. Among, such materials are the various metals, metalloids and ceramics.

A fundamental consideration in the success of this method is that the melt must be extruded into a gaseous atmosphere capable of reaction with it. In practice, the hot filamentary jet issuing from the extrusion orifice contacts the atmosphere with a resulting immediate reaction which forms a film about the jet surface. This film, called the stabilizing film, stabilizes the filamentary jet or stream against break-up from the forces of surface tension until sufficient heat can be dissipated to effect a phase change from the molten to the solid state.

Although the intended purpose of the afore-noted technique is to provide a capability for extruding fibers and filaments from extremely low viscosity melts, it has now been discovered that with appropriate modification certain elements of the technique can be employed to great advantage in the production of composite metal wire, and particularly aluminum-clad steel wire.

In brief, the method of this invention may be carried out by first threading a solid steel wire so that it is made to pass directly through the orifice center of a melt extrusion unit then into a gas retaining column and finally on to a wind-up device. Once the steel core wire has been properly positioned, solid aluminum is then placed in the crucible of the extrusion unit and sufficient heat is supplied to bring it to the melt. By means of an inert gas head pressure, extrusion of the melt is begun and the steel wire, which has been preheated to the temperature of the melt, is simultaneously advanced through the center of the orifice. From the extrusion orifice, the steel wire and surrounding aluminum melt exit into an atmosphere containing a gas highly reactive with aluminum. This results in immediate reaction and a so-called stabilizing film is caused to form about the surface of the hot, molten aluminum. This film or protective sheath serves to stabilize the structural integrity of the molten aluminum coating from surface tension disintegration until the solid state can be attained through sufficient cooling.

The wire product obtained by this procedure is characterized by an outer layer of aluminum having an exceptionally smooth surface bonded concentrically to an inner core of steel. The depth of aluminum in the com posite is controllable and can be made to exceed the steel core in cross-sectional area. That is, a composite wire can be fabricated in which aluminum is themajor component in the total cross-section of the wire composite. Although the exact mechanism of the bonding is not known with certainty, it is believed that it proceeds from the initial formation of an alloy at the steelaluminum interface.

DESCRIPTION OF THE DRAWINGS As indicated, FIG. 1 is a schematic representation in vertical crosssection of a general type extrusion apparatus which may be employed in the practice of the present invention. The head 10 of the apparatus serves to contain the pressurized inert gas used during opera: tion to force the melt through the extrusion orifice and is supplied to the system by piping means (not shown). The unit is further equipped with a melt crucible 11, which is generally heated by means of induction coils (not shown). Positioned within the crucible at the lower extremity thereof is a hearth plate l2'upon which the molten aluminum 13 is supported. Mounted within the hearth plate is a cylindrical tube 14 through which the steel wire 15 is threaded. Tube 14 functions to separate the steel wire 15 from contact with the molten aluminum inside the crucible and also provides a guide means for centering the wire. A controlled exit of the aluminum melt from the crucible is permitted by means of plug valve 16 which is suitably positioned in hearth plate 12. The plug valve may be engaged or disengaged from outside the apparatus by means of wire attachment 17. When disengaged, the molten aluminum is allowed to drop from the crucible by gravity into barrel reservoir 18, which is fitted at its lower end with an orifice assembly 19. The aluminum melt is then forced through orifice assembly 17 by means of an inert gas head pressure which is exerted on the melt in crucible 11 and transmitted to reservoir 18 through tube 14 which interconnects these two components. While the molten aluminum is being extruded, steel wire 15 is simultaneously forwarded through the center of the orifice. At the orifice exit the aluminum coated steel wire contacts an oxygen containing gaseous atmosphere, provided by piping means (not shown), and then passes downward through cooling column 20. Positioned beneath column 20 are godets 21 and 22 which guide the aluminumclad steel wire product to traversing guide 23 from which it is placed on windup device 24.

In operation, steel wire is supplied to the system from bobbin 25 with guide 27 and sealed guide 28 serving to properly position the wire feed as it enters the apparatus. Before contacting the molten aluminum in barrel reservoir 18, the wire is preheated to the temperature of the melt by sliding over rod contact 29. An electrical potential is thus picked up by the steel wire which is grounded when the heated wire reaches the aluminum melt pool in reservoir 18. The voltage supplied to rod contact 29 is controlled by means of variac 30. Tension control on the wire as it passes through the process is controlled by magnetic clutch 26. This device also prevents unwinding of bobbin 25 when the operation is slowed or stopped.

The hearth plate 12 in FIG. 1 together with associatd cylindrical tube 14 and plug valve 16 are shown in greater detail in the enlarged view of FIG. 2. The numeral 33 designates the central aperture of tube 14 which communicates with the top end of barrel reservoir 18 in FIG. 1. The numeral 31 denotes the plug element of plug valve 16, which is operated by means of wire attachment 17. When the plug is raised molten aluminum is allowed to drop by gravity flow into barrel reservoir 18 (FIG. 1) through passageway 32.

FIG. 3 depicts in detail an orifice assembly which may be suitably employed in the apparatus of FIG. 1. The assembly is generally indicated by the numeral 19 and comprises two concentric plates 34 and 35 stacked one on the other with a gap space between them. Each plate contains a centrally disposed orifice with the orifice of one plate being in co-axial alignment with that of the other. Shown moving through the orifices is steel wire core 15 supported by molten aluminum 13. A reactive gas containing atmosphere 36, supplied through the gap space between the orifice plates 34 and 35 makes contact with the molten aluminum surface as it exits from the orifice of plate 34.

FIG. 4 illustrates an alternative orifice assembly to that illustrated in FIG. 3 and may likewise be employed with advantage in the apparatus of FIG. 1. The assembly is comprised of three concentric plates 34, 38 and 35 as opposed to the two shown in FIG. 3. As in FIG. 3, the plates are in stacked relationship with each containing a centrally disposed orifice in co-axial alignment one with the other. There is a gap space between plates 34 and 38 with a second such gap being present between plates 38 and 35. An inert gas, e.g., helium, is supplied to the space between plates 34 and 38; while a reactive gas is fed into the gap space between plates 38 and 35. The purpose of the inert gas is to shield the orifice of plate 34 from the deteriorating effects of exposure to the reactive gaseous atmosphere. As in FIG. 3, the numeral 15 denotes the steel wire 15 and surrounding aluminum melt 13 moving through the orifice assembly.

DESCRIPTION OF THE INVENTION A detailed description of the sequential steps employed in the method of this invention is set forth hereafter Throughout the description references are made to the appended drawings to facilitate understanding.

In preparation for start up, steel wire from a supply bobbin is first positioned within the process apparatus, such as is depicted in FIG. 1. That is, the steel wire is first threaded into the head 10 of the extrusion unit, then through tube 14 and barrel reservoir 18 and thence through the orifice assembly 19. From the orifice exit, the wire is passed downward through cooling column 20, then over guides 21 and 22 to transverse guide 23 and finally on to windup 24.

After proper positioning of the steel core wire, aluminum bar stock is placed upon the hearth plate 12 within crucible 11. This is followed by the introduction of a pressurized inert gas into the head 10 of the extrusion unit via piping means (not shown in the drawing). At this point, the heat cycle is begun using induction coils wrapped about the crucible, or any other convenient heat source.

During the heating cycle, the steel wire is slowly moved through the apparatus to prevent excess heating or reaction of the wire in the area of the crucible 11. In the course of the heating cycle and subsequent processing, the steel wire core is shielded from direct contact with the aluminum in the area above the hearth plate 12 by virtue of protective tube 14 through which it has been threaded. The reason for this separation is that the wire tends to pick up aluminum as it passes through the melt. This acquired coating of molten aluminum has a tendency to ball on the wire surface and to solidify thereon in this form. As might be expected, when such protrusions impinge against the extrusion orifice the result is a break in the moving wire. Such ball formation usually occurs when the steel wire is allowed to contact molten aluminum above the hearth plate. In this area the surface tension forces on the molten aluminum coating are free to operate unabated. The reason is that there has not yet been contact with the reactive gaseous atmosphere which forms a thin film about the aluminum surface and serves to prevent a surface tension induced disintegration of the coating. Such film formation occurs further downstream at the extrusion orifice 19.

After the aluminum in crucible 11 has been brought to the melt, the system is taken to the operating temperature for extrusion. The temperature employed is generally from about 20 to 40 C. above the melting point of aluminum (m.p. 660 C.) to allow for any uncontrollable heat losses during extrusion. Generally, a temperature of from about 670 to 700 C, has been found adequate. The moving wire is also brought to an approximation of the operating temperature to minimize the withdrawal of heat as the wire moves through the pool of aluminum melt in reservoir 18. Without preheating, the wire tends to extract heat in such quantity as to causes some freezing in the melt pool. This, of course, significantly impairs the capability for a satifactory melt extrusion. Preheating is accomplished by permitting the steel wire to slide over rod contact 29 where it picks up an electrical potential which is grounded as the wire enters the melt pool in barrel reservoir 18. It has been found that approximately 100 watts of electrical energy will supply the heat necessary to raise a 4 mil wire to 700 C. from room temperature when moving at a rate of 1,000 feet per minute.

In the next sequence of steps, the system is pressurized to the level desired for extrusion, the plug valve 16 is disengaged to allow passage of the aluminum melt into barre] reservoir 18, the steel wire is brought to the desired velocity in its downward movement through the orifice center and extrusion of the melt is begun. Within the orifice assembly 19 (see FIG. 3), the molten aluminum surrounding the steel wire is caused to contact a gaseous atmosphere which is reactive with it. Reaction occurs almost instantaneously with the result that a thin film is formed over the entire surface of the molten aluminum coating. This film stabilizes the surface tension induced degeneration of the molten coating (e.g., balling) until sufficient heat is transferred to effect solidification. Solidification of the aluminum occurs in the course of movement downstream through cooling column 20. The steel core-aluminum matrix wire which emerges from the cooling column is then taken up on a windup mechanism 24. In those instances where the removal of relatively large quantities of heat is required e.g., during production of large diameter wire in which the aluminum matrix constitutes the greater portion of cross-sectional areait may be desirable to interpose a water spray system between the air cooling column 20 and take-up device 24.

The reactive gaseous atmosphere supplied to the orifice area so as to provide the extruded aluminum with a protective film or sheath until solidification can occur should meet certain requirements. That is, the film formed by reaction of the gas with the aluminum should be relatively insoluble in the alumimum melt; e.g., the film solubility should not exceed percent by weight. Moreover, the film formed must possess a melting point higher than that of the stream material if stabilization is to occur. Also, the gaseous atmosphere must be capable of a very high rate of reactivity with molten aluminum. Oxygen, which forms an aluminum oxide film has been found suitable. Another gas which may be used is ammonia resulting in the formation of an aluminum nitride film. A still further selection is hydrogen sulfide, which forms an aluminum sulfide film. The rate at which the reactive gaseous atmosphere is supplied to the orifice area cannot be specified with particularity, since it will vary widely with changes in operating conditions. For example, it will vary with the reactivity of the particular gas used, the diameter of the composite wire being produced, the thickness of the aluminum matrix in the wire under production, the production rate being employed, the operating temperature and other factors. The desired rate of gas flow under the conditions of any given run can readily. be dewhere D is the total diameter desired 'in mils of the composite wire to be produced; d is the diameter of the steel wire in mils; C is the orifice coefficient, D the diameter of the orifice in mils; AP is the net activation pressure (head pressure) in pounds per square inch; p is the density of the clad material in gms. per cc; and S is the speed of the wire windup in feet per minute. For an aluminum cladding system in which the orifice diameter is in the range of 30 mils, the equation reduces to the following form;

Although it is not possible to set forth precise limits on the range of operability for either head pressure or wire velocity because of the many variables involved, including variation in equipment design such as cooling capacity; in general, applicable head pressures will fall within the range of from 0.5 to 20 p.s.i.g. with wire velocities being within the range of from about 200 to 1,500 feet per minute.

Reference shall now be had to the following examples which are to be taken as illustrative, but not limitative of the principles and practice of the present invention and in which apparatus as represented in the drawings was employed.

EXAMPLE 1 Steel core wire having a diameter of 6 mils was first threaded through the extrusion apparatus according to the procedure as set forth above followed by the placement of aluminum bar stock upon the hearth plate within the crucible of the apparatus. The heat cycle was then begun during which time the steel wire was moved slowly through the apparatus to avoid excess localized heating of the wire in the crucible area.

After the solid aluminum had been converted to a molten condition, the extrusion unit was brought to an operating temperature of about 690 C. and pressurized, i.e., pressurized argon gas was introduced into the system to effect a head pressure of 23 inches water gauge over the melt pool of aluminum. The steel wire was then brought to a forwarding speed of 300 feet per minute during the course of which it was subject to a preheating to the operating temperature before making contact with the aluminum melt. The plug in the hearth plate was released and the aluminum melt passed into the reservoir from where it could be ejected through the extrusion orifice by means of the applied head pressure. At this point, the molten aluminum extruded through the orifice, which measured 30 mils in diameter; while the steel wire was being passed through the orifice center at a rate of 300 feet per minute. During extrusion, inert helium gas was supplied across the bottom side of the orifice plate at a rate of 1.3 liters per minute so as to blanket the orifice and prevent the erosion thereof by contact with reactive oxygen used as a film stabilization gas. To effect film stabilization pending solidification, oxygen was supplied to the orifice assembly at the rate of 0.8 liter per minute. Cooling of the molten aluminum which surrounded the steel core wire was effected by passage through the air cooling column and thence through a water spray positioned at the column exit. The resulting steel-aluminum composite wire has a cross-sectional diameter of mils.

EXAMPLE 2 Again employing steel core wire with a diameter of 6 mils, a run similar to that of Example 1 was made to produce a composite steel-core aluminum wire product having a total diameter of mils. The extrusion was conducted through a shaping orifice measuring 34 mils in diameter under operating conditions as follows: Temperature 675 C; head pressure 17.7 inches of water gauge; wire speed through the orifice 400 feet per minute; helium flow rate across the orifice plate 1.3 liter per minute; and flow rate of reactive oxygen gas to the orifice assembly 0.8 liter per minute.

It will occur to those skilled in the art to which this invention pertains that modifications can readily be made to the particular embodiments of the invention as described hereinabove. However, it is to be understood that such modifications in general will not depart from the spirit and scope of this invention as defined by the following claims.

1 claim:

1. A method for producing aluminum-clad steel composite wire comprising the steps of:

a. forming a clad of molten aluminum about a preheated strand of steel wire by moving the steel wire directly through the center of a melt extrusion orifice whie simultaneously extruding molten aluminum through said orifice, said steel wire having been preheated to a temperature proximate to that of the molten aluminum before contact therewith;

b. causing a protective film to form about said molten aluminum clad by contacting same with a reactive gaseous atmosphere immediately upon exit from said melt extrusion orifice; and

c. cooling the molten aluminum-clad steel wire to effect a solidification of the aluminum cladding.

2. The method of claim 1, wherein said reactive gaseous atmosphere is oxygen.

3. The method of claim 1, wherein said reactive gaseous atmosphere is ammmonia.

4. The method of claim 1, wherein said reactive gaseous atmosphere is hydrogen sulfide.

5. A method for producing aluminum-clad steel composite wire comprising the steps of:

a. forming a clad of molten aluminum about a strand of steel wire preheated to a temperature in the range of about 670 to 700 C. by forwarding said steel wire directly through the center of a melt extrusion orifice at a velocity in the range of from about 200 to 1,500 feet per minute while molten aluminum is being concurrently extruded through said orifice under a head pressure of from about 0.5 to 20 p.s.i.g. and a temperature in the range of about 670 to 700 C.;

b. causing a protective film to form about said molten aluminum clad by contacting same with a reactive gaseous atmosphere immediately upon exit from said melt extrusion orifice; and

c. cooling the molten aluminum-clad steel wire to effect solidification of the aluminum cladding.

6. The method of claim 1, wherein said reactive gaseous atmosphere is oxygen. 

1. A METHOD FOR PRODUCING ALUMINUM-CLAD STEEL COMPOSITE WIRE COMPRISING THE STEPS OF: A. FORMING A CLAD OF MOLTEN ALUMINIUM ABOUT A PREHEATED STRAND OF STEEL WIRE BY MOVING THE STEEL WIRE DIRECTLY THROUGH THE CENTER OF A MELT EXTRUSION ORIFICE WHILE SIMULTANEOUSLY EXTRUDING MOLTEN ALUMINIUM THROUGH SAID ORIFICE, SAID STEEL WIRE HAVING BEEN PREHEATED TO A TEMPERATURE PROXIMATE TO THAT OF THE MOLTEN ALUMINUM BEFORE CONTACT THEREWITH; B. CAUSING A PROTECTIVE FILM TO FORM ABOUT SAID MOLTEN ALU MINUM CLAD BY CONTACTING SAME WITH A REACTIVE GASEOUS ATMOSPHERE IMMEDIATELY UPON EXIT FROM SAID MELT EXTRUSION ORIFICE; AND C. COOLIG THE MOLTEN ALUMINUM-CLAD STEEL WIRE TO EFFECT A SOLIDIFICATION OF THE ALUMINUM CLADDING.
 2. The method of claim 1, wherein said reactive gaseous atmosphere is oxygen.
 3. The method of claim 1, wherein said reactive gaseous atmosphere is ammmonia.
 4. The method of claim 1, wherein said reactive gaseous atmosphere is hydrogen sulfide.
 5. A method for producing aluminum-clad steel composite wire comprising the steps of: a. forming a clad of molten aluminum about a strand of steel wire preheated to a temperature in the range of about 670* to 700* C. by forwarding said steel wire directly through the center of a melt extrusion orifice at a velocity in the range of from about 200 to 1,500 feet per minute while molten aluminum is being concurrently extruded through said orifice under a head pressure of from about 0.5 to 20 p.s.i.g. and a temperature in the range of about 670* to 700* C.; b. causing a protective film to form about said molten aluminum clad by contacting same with a reactive gaseous atmosphere immediately upon exit from said melt extrusion orifice; and c. cooling the molten aluminum-clad steel wire to effect solidification of the aluminum cladding.
 6. The method of claim 1, wherein said reactive gaseous atmosphere is oxygen. 